1. Field of the Invention
This invention relates generally to the use of Group 13 element compounds to improve the productivity of ionic transition metal catalyst systems. More specifically the invention relates to the use of a Group 13 element organometallic compound as an additive to an ionic mono(cyclopentadienyl)/heteroatom transition metal olefin polymerization catalyst system. These catalyst systems have enhanced productivity over similar catalysts, in the absence of Group 13 compounds, for the polymerization of unsaturated monomer, e.g., olefinically unsaturated monomers such as ethylene, other ethylenically unsaturated olefins or olefinic species, diolefins, cyclic olefins, ethylenically unsaturated non-cyclic non-conjugated polyenes, cyclic non-conjugated polyenes, and acetylenically unsaturated monomers or combinations thereof, to form polymers such as polyolefins having narrow molecular weight distributions.
2. Background
Ziegler-Natta type catalysts for the polymerization of olefins are well known. The traditional Ziegler-Natta type systems comprise a metal halide activated to a catalyst species by reaction with a metal alkyl cocatalyst, particularly an aluminum alkyl cocatalyst. The activation of these traditional heterogeneous Ziegler-Natta catalysts generates a variety of different active sites. As a consequence of this non-uniformity of the active sites, the catalysts produce polymer products of broad molecular weight distribution (MWD). In many application, this broad MWD is undesirable. Furthermore, the polymer products exhibit relatively poor composition distribution (CD), comonomer incorporation and sequence distribution.
Also known are catalyst systems comprising a bis(cyclopentadienyl) ("Cp") transition metal and a compound cocatalyst. Such bis Cp transition metal compounds, in the presence of activators catalyze the polymerization of olefin monomers to polyolefins. Bis (Cp) transition metal compounds of the Group 4 metals, particularly of titanium and zirconium, have been used. When such transition metal components are cocatalyzed with an aluminum alkyl--the cocatalyst used with traditional Ziegler-Natta catalysts--the catalytic activity is generally to low for commercial applications.
From EP-A-129368 (Exxon) it is known that bis (Cp) transition metal components may be activated by alumoxanes. The bis(Cp) transition metal compound-alumoxane catalysts systems, whether homogeneous or supported, are versatile in that they may have a range of activities from low to high and may be used effectively to produce a variety of polymer products including, for example, low molecular weight copolymers high density linear polyethylene (HDPE), linear low density polyethylene (LLDPE), ethylene-propylene copolymer (EP), atactic polypropylene (a-PP), syndiotactic (s-PP) polypropylene and isotactic polypropylene (i-PP). The bis (Cp) transition metal compound-alumoxane catalyst systems also offer a significant advantage over the traditional Ziegler-Natta catalysts in that they are able to produce polymers with narrow MWD.
EP-A-0277003 (Exxon) and EP-A-0277004 (Exxon) teach a catalyst systems comprising a transition metal component and a non-coordinating anion. Such catalyst systems may be formed by activating the transition metal compound to a catalytic state by reaction with certain types of ionic exchange compositions.
U.S. Pat. No. 5055438 (Exxon), incorporated herein by reference, EP-A-420436 (Exxon), WO 91/04257 (Exxon) and EP-A-416815 (Dow) describe a further class of catalysts having a single cyclopentadienyl ring ligand and a heteroatom bonded to the metal atom, and their use in conjunction with alumoxanes. Similar catalyst systems are taught in U.S. Pat. No. 5,064,802 (Dow), incorporated herein by reference, EP-A-418044 (Dow) and WO 92/00333 (Exxon), but having a non-coordinating anion as activating cocatalyst. Here the catalyst system is prepared as a reaction product of a mono(cyclopentadienyl) heteroatom metal compound and an ionic activator compound. Such catalyst systems referred to as an "ionic mono (Cp)/heteroatom transition metal catalyst" or an "ionic mono(Cp) catalyst" permits the production of polyolefin products of narrow MWD at high rates of catalytic activity with good incorporation of comonomers and control of the chain end chemistry of the polymer products.
The active catalytic ion pair species of the ionic mono (Cp) catalyst may be irreversibly inactivated by Lewis base impurities contained in components of the polymerization medium such as the diluent or the monomer supply with which the ionic mono (Cp) catalyst is used. Despite the most elaborate control some, albeit minute, quantity of such Lewis base impurities will invariably be present in the polymerization medium, especially the commercial monomer supply. Therefore there is a need for an additive effective for use during polymerization to neutralize impurities contained in the polymerization medium without affecting the ability of these ionic mono (Cp) catalysts to produce polyolefin products.